The vast majority of infectious agents are encountered at mucosal surfaces. Understanding the dynamic interface of immune cells and infectious microbes at these sites is one of the greatest challenges in the fields of microbiology and immunology. These interactions are especially important in neonatal life when microbial antigens are encountered for the first time. To study immune cell/bacterial interactions during the neonatal period, we have developed a novel murine model of orogastric infection with the extracellular bacterium Yersinia enterocolitica. Strikingly, neonates are markedly more resistant than adults to this enteropathogen, an observation unique among all other studied neonate-pathogen interactions. This system provides an unprecedented opportunity for learning how fully protective immunity is achieved in the neonatal intestine. Thus, a major goal of this application is to identify the eukaryotic immune mechanisms leading to resistance of neonates to intestinal exposure to Y. enterocolitica. A second major goal of this application is to apply Y. enterocolitica products to the regulation of neonatal intestinal immunity in health and disease. These defined goals will provide major new building blocks for our long term goals of (a) understanding how intestinal immunity develops during ontogeny, (b) achieving highly effective pediatric mucosal vaccines, and (c) developing novel strategies for the treatment of pathological intestinal inflammation in early life. Specific Aim 1 will define adaptive immune mechanisms underlying protection of neonates from oral Y. enterocolitica infection. The effects of neutralization of ?IFN and/or IL-17 responses on inflammatory cell infiltration, bacteremia, and relative weight loss associated with mortality will be examined. The importance of B cell responses will be tested by genetic and acute B cell ablation and in B cell-sufficient neonates unable to mount bacterial-specific antibody responses. Lastly, the contribution of natural Treg (CD4+CD25+) will be analyzed by adoptive transfer of adult Treg to infected neonates and by the in vivo depletion of the Treg compartment. Specific Aim 2 will exploit the Y. enterocolitica effector protein YopP (its presence or its absence) to manipulate neonatal immune responses and identify key target immune cell types in vivo. Since the ?YopP mutant strain induces profound inflammation in the neonatal intestine, we will test the capacity of a modified ?YopP mutant strain to act as a potent adjuvant for mucosal immunization. An ovalbumin - expressing ?YopP strain will be created and ovalbumin-specific antibody and Th responses will be monitored, as well as the in vivo activation of ova-specific TCR transgenic T cells. Second, the possibility that YopP, by itself, can downregulate pathogenic intestinal inflammation in neonates will be tested. Bacteria (Y. enterocolitica and Salmonella) engineered to express only YopP will be applied to modulate inflammation in DSS-induced colitis in neonates. In addition, cell types critical for Y. enterocolitica pathogenesis will be identified by assessing NF?B activation in intestinal cells following infection with the ?YopP strain. Children are highly susceptible to microbes transmitted through contaminated food. This application has two main objectives. First, we will discover how the intestinal immune system in early life develops protective responses against microbes;second, we will learn how products of the microbes themselves can be used to enhance childhood vaccine responses and treat intestinal inflammation, such as pediatric Crohn's disease.